按组条件选择记录

Question

我是Python新手（使用anacondaw/pythonv3.4.3），还没有找到这个答案，但这似乎是一个非常重要的问题，我一定是走错了方向

import pandas as pd
url = 'https://raw.github.com/pydata/pandas/master/pandas/tests/data/tips.csv'
tips = pd.read_csv(url)
tips.head(5)
Out[1]:
   total_bill   tip     sex smoker  day    time  size
0       16.99  1.01  Female     No  Sun  Dinner     2
1       10.34  1.66    Male     No  Sun  Dinner     3
2       21.01  3.50    Male     No  Sun  Dinner     3
3       23.68  3.31    Male     No  Sun  Dinner     2
4       24.59  3.61  Female     No  Sun  Dinner     4

我想选择day组中至少有50条记录的记录

sel_days = tips.groupby("day").size() > 50
sel_days
Out[2]: 
day
Fri     False
Sat      True
Sun      True
Thur     True
dtype: bool

我看到这是一个序列，但似乎不知道如何生成一个布尔序列来从原始集合tips中选择行

type(sel_days)
Out[3]: pandas.core.series.Series
print(x in sel_days for x in tips["day"])
<generator object <genexpr> at 0x0000000007DBDFC0>

我该怎么做

Answer 1

我将向映射布尔掩码的tips数据帧添加一个新列：

tips['mask'] = tips['day'].map(sel_days)

然后仅选择真值：

tips = tips[tips['mask']]

Answer 2

您想^{}：

In [22]:
tips.groupby('day').filter(lambda x: len(x) > 50)

Out[22]:
     total_bill   tip     sex smoker   day    time  size
0         16.99  1.01  Female     No   Sun  Dinner     2
1         10.34  1.66    Male     No   Sun  Dinner     3
2         21.01  3.50    Male     No   Sun  Dinner     3
3         23.68  3.31    Male     No   Sun  Dinner     2
4         24.59  3.61  Female     No   Sun  Dinner     4
5         25.29  4.71    Male     No   Sun  Dinner     4
6          8.77  2.00    Male     No   Sun  Dinner     2
7         26.88  3.12    Male     No   Sun  Dinner     4
8         15.04  1.96    Male     No   Sun  Dinner     2
9         14.78  3.23    Male     No   Sun  Dinner     2
10        10.27  1.71    Male     No   Sun  Dinner     2
11        35.26  5.00  Female     No   Sun  Dinner     4
12        15.42  1.57    Male     No   Sun  Dinner     2
13        18.43  3.00    Male     No   Sun  Dinner     4
14        14.83  3.02  Female     No   Sun  Dinner     2
15        21.58  3.92    Male     No   Sun  Dinner     2
16        10.33  1.67  Female     No   Sun  Dinner     3
17        16.29  3.71    Male     No   Sun  Dinner     3
18        16.97  3.50  Female     No   Sun  Dinner     3
19        20.65  3.35    Male     No   Sat  Dinner     3
20        17.92  4.08    Male     No   Sat  Dinner     2
21        20.29  2.75  Female     No   Sat  Dinner     2
22        15.77  2.23  Female     No   Sat  Dinner     2
23        39.42  7.58    Male     No   Sat  Dinner     4
24        19.82  3.18    Male     No   Sat  Dinner     2
25        17.81  2.34    Male     No   Sat  Dinner     4
26        13.37  2.00    Male     No   Sat  Dinner     2
27        12.69  2.00    Male     No   Sat  Dinner     2
28        21.70  4.30    Male     No   Sat  Dinner     2
29        19.65  3.00  Female     No   Sat  Dinner     2
..          ...   ...     ...    ...   ...     ...   ...
207       38.73  3.00    Male    Yes   Sat  Dinner     4
208       24.27  2.03    Male    Yes   Sat  Dinner     2
209       12.76  2.23  Female    Yes   Sat  Dinner     2
210       30.06  2.00    Male    Yes   Sat  Dinner     3
211       25.89  5.16    Male    Yes   Sat  Dinner     4
212       48.33  9.00    Male     No   Sat  Dinner     4
213       13.27  2.50  Female    Yes   Sat  Dinner     2
214       28.17  6.50  Female    Yes   Sat  Dinner     3
215       12.90  1.10  Female    Yes   Sat  Dinner     2
216       28.15  3.00    Male    Yes   Sat  Dinner     5
217       11.59  1.50    Male    Yes   Sat  Dinner     2
218        7.74  1.44    Male    Yes   Sat  Dinner     2
219       30.14  3.09  Female    Yes   Sat  Dinner     4
227       20.45  3.00    Male     No   Sat  Dinner     4
228       13.28  2.72    Male     No   Sat  Dinner     2
229       22.12  2.88  Female    Yes   Sat  Dinner     2
230       24.01  2.00    Male    Yes   Sat  Dinner     4
231       15.69  3.00    Male    Yes   Sat  Dinner     3
232       11.61  3.39    Male     No   Sat  Dinner     2
233       10.77  1.47    Male     No   Sat  Dinner     2
234       15.53  3.00    Male    Yes   Sat  Dinner     2
235       10.07  1.25    Male     No   Sat  Dinner     2
236       12.60  1.00    Male    Yes   Sat  Dinner     2
237       32.83  1.17    Male    Yes   Sat  Dinner     2
238       35.83  4.67  Female     No   Sat  Dinner     3
239       29.03  5.92    Male     No   Sat  Dinner     3
240       27.18  2.00  Female    Yes   Sat  Dinner     2
241       22.67  2.00    Male    Yes   Sat  Dinner     2
242       17.82  1.75    Male     No   Sat  Dinner     2
243       18.78  3.00  Female     No  Thur  Dinner     2

[225 rows x 7 columns]